Thanks, Ben, for the dhclient.conf example.  I believe I've figured out
what the problem is, though.  My /etc/ipnat.conf refers to an outdated
IP address for my external interface.  Now I'm not sure why routing
was working at all, even for a few minutes!

Is there a reliable way to update the IP in ipnat.conf automatically if
an interface gets a new address via dhcp?

Thanks.

Phil Brodd